Since personal computers have been commercially introduced, due to their compactness, diversified applications, and because their prices are acceptable by the public, they have become popular among individuals, families, firms, and the like. Conventionally, in assembling the casing of the computer system unit, it is assembled by means of screws, making opening of the casing rather troublesome and time-consuming when servicing of the computer is required. For example, there is shown in FIG. 5 a conventional computer casing comprising an upper box 51 and a bottom fox 52 assembled in such a way that the lower edges 53 of the upper box 51 are pushed to be embedded along the recesses 54 of the bottom box and then screws 55 are threaded through the holes 57 on the bottom box 52 to be locked into the threaded holes 56 on the upper box 51. In this case, five screws have to be screwed-in and screwed-out in order to open and close the upper box body, which is time-consuming.